Electroplating method for sealing liquid-cooled generator stator bar structures

ABSTRACT

The interior of a stator bar clip that is joined to the end of a stator bar is electroplated so that a metallic barrier coating overlies the braze joint between the stator bar and the clip to define a seal at and around the joint that is substantially impermeable to liquid.

BACKGROUND OF THE INVENTION

Water-cooled stator bars for electrical generators are comprised of aplurality of small rectangular solid and hollow copper strands brazed toone another to form a bar. The ends of the strands are brazed to an endfitting, typically referred to as stator bar clip. A cover is brazed tothe clip window. The end fitting serves as both an electrical and acooling flow connection for the stator bar.

The hollow end fitting typically includes an enclosed chamber foringress or egress of stator bar cooling liquid, typically deionizedwater. At one end, the end fitting receives the ends of the strands ofthe stator bar. The fitting and the peripherally outermost copperstrands of the stator bar are brazed to one another. The opposite end ofthe fitting is connected to a stator cooling conduit.

Liquid cooled stator bar clips have gone through design changes over theyears. However, they typically contain mixed solid and hollow strandsbrazed to one another, and a cover brazed to a clip window. Duringoperation, the hollow strands carry water to cool off the bar. Overtime, leaks can develop about the connection between the stator bar endsand the stator bar fitting, between cover and clip as well as betweenadjacent strands. Leaks may also occur at various plumbing connections.It is believed that the major leak mechanism is a crevice corrosionprocess which initiates in the braze alloy at the interior surface ofthe braze joint. Crevice corrosion is a localized form of corrosionusually associated with a stagnant solution on the micro-environmentallevel. Such stagnant microenvironments tend to occur in crevices such asmicro surface voids formed during brazing, especially at the boundary ofstrands and braze alloy. Crevice corrosion is initiated by changes inlocal chemistry within the crevice, such as shift to phosphorous acidconditions in the crevice. Stagnant water in the chamber of the fittingis in contact with the braze alloy and the copper strands. This coolantcontact with the braze joint and cooper strands is believed to causecorrosion and consequent leakage.

Field repair of coolant leaks through the stator bar end connections hasbeen successful. A leak site is identified by several different tests,such as vacuum decay and traceable Helium test.

An epoxy barrier coating method has been used as a leak repair andprevention method. An example of an epoxy barrier coating method isdisclosed in U.S. Pat. No. 5,605,590, the disclosure of which isincorporated herein by this reference. This epoxy barrier coating hasbeen applied to provide protection against water initiated corrosionmechanisms along the brazed length of the strand package. Epoxy coatingis manually injected. The voids and air pockets are possible duringinjection. Thus, the process is labor intensive and requires 100%inspection. As a result, the process can be labor intensive, takes along time to complete, and can produce defects.

There are also other leak issues in liquid cooled generators. Indeed,there are many types of leaks associated with water-cooled generators.Stator bar end crevice corrosion is the major one. But other leaks arecaused by porosity, cracking and localized damage during manufacturingrather than as a result of corrosion.

There is a need for an improved method for leak repair and prevention inliquid-cooled generators. In particular, there is a need for corrosionprotection at the junction between the stator bars and their clips. Thecorrosion protection should be robust and be applicable to variousstator bar clip designs, including clips for recessed braze, flush brazeand raised hollow strand braze designs. There is also a need for animproved method to repair and prevent leaks within the generator waterpass that arise from other causes.

BRIEF DESCRIPTION OF THE INVENTION

The invention proposes an electroplating method to deposit a thin layerof metallic barrier coating on a part of or the entire interior surfaceof a liquid-cooled stator bar clip. The metallic layer provides acorrosion resistant barrier coating to prevent water access to corrosionsusceptible region(s) such as clip-to-strand braze joints. Differentmaterials may be deposited, such as copper or nickel as the metallicbarrier coating.

Thus, the invention may be embodied in an apparatus for electroplatingan interior of a component, comprising: an inflow tube for flowingelectroplating solution to an interior of said component; an outflowtube for flowing electroplating solution out of an interior of saidcomponent; a pump for conducting electroplating solution through saidinflow tube and for drawing said electroplating solution through saidoutflow tube; an anode disposed in said electroplating solution andelectrically connected to a power supply; and said component beingelectrically connected to said power supply as a cathode.

The invention may also be embodied in a method of sealing a stator barwith an interior liquid pass for flow of coolant forming at least aportion of said stator bar, comprising electroplating at least a portionof an interior wetted surface of said stator bar.

The invention may also be embodied in a method of sealing interiorsurfaces of a fitting receiving a stator bar end, to define a sealsubstantially impermeable to liquid, the fitting having a chamber forreceiving a liquid through an opening in the fitting and said chamberbeing in communication with hollow strands forming at least a portion ofsaid stator bar, said method comprising the steps of: disposing anelectroplating solution in said chamber; disposing an anode of the metalor metal alloy to be deposited in said electroplating solution;connecting said anode to the positive terminal of a current source as acathode; electrically connecting said stator bar to the negativeterminal of said current source; and establishing an electricalpotential between said anode and said cathode to initiateselectrophoretic migration of metal ions from the anode to theelectrically conductive inner surface of the component.

The invention is also embodied in stator bar end and a fitting receivingthe end to define a seal substantially impermeable to liquid, thefitting having a chamber for receiving a liquid through an opening inthe fitting and in communication with hollow strands forming at least aportion of said stator bar and for flow of the liquid through the hollowstrands, wherein at least a portion of an interior surface of saidfitting has a metallic layer disposed thereon, wherein said metalliclayer is deposited by electroplating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a liquid-cooled stator windingarrangement illustrating the stator bars and end fittings coupled toinlet and outlet coolant headers;

FIG. 2 is a schematic cross-section of a plating set-up for plating theinterior of a component, such as a stator bar clip; and

FIG. 3 is a schematic illustration of a plating set-up for simultaneousplating wherein the bar is flooded with circulating solution.

DETAILED DESCRIPTION OF THE INVENTION

Water-cooled stator bars for electrical generators are comprised of aplurality of small rectangular solid and hollow copper strands which arebrazed to one another and brazed to an end fitting. The end fittingserves as both an electrical and a hydraulic connection for the statorbar. The end fitting typically includes an enclosed chamber for ingressor egress of stator bar cooling liquid, typically deionized water.Another opening of the end fitting receives the ends of the strands ofthe stator bar, the fitting and peripherally outermost copper strands ofthe stator bar being brazed to one another. Over time, leaks havevariously developed about the connection between the stator bar ends andthe stator bar fitting as well as between adjacent strands. It isbelieved, based on leak analysis results, that the leak mechanism is dueto a crevice corrosion process which initiates in the braze alloy at theinterior surface of the braze joint. Crevice corrosion is initiated bychanges in local chemistry within the crevice, such as shift tophosphorous acid conditions in the crevice.

There are also other leak issues in liquid cooled generators. Examplesof other leaks in addition to the clip crevice corrosion leaks are clipcrack leaks, clip window leaks, plumbing and fitting leaks, andconnection ring leaks.

Field repair of leaks through the stator bar end connections has onlybeen moderately successful.

In an embodiment of the invention, an electroplating method is proposedto deposit a thin layer of metallic barrier coating on the entireinterior surface of liquid-cooled stator bars' water pass, orselectively on target areas, e.g., with high corrosion risks orotherwise having a high leak potential. The metallic layer provides acorrosion resistant barrier coating to prevent water access to corrosionsusceptible region(s) such as clip-to-strand braze joints. Concurrentlythe coating has the capability to repair and prevent substantially allother types of leaks.

Referring now to the drawings, particularly to FIG. 1, there isillustrated a liquid-cooled stator winding arrangement used in a typicalliquid-cooled generator. A stator core having stator core flanges 12 andcore ribs 14 is illustrated, with stator bars 16 passing throughradially extending slots and terminating at opposite ends in endfittings 18 and 20. Inlet hose 22 connects inlet fitting 18 to an inletcoolant header 24 and outlet hose 26 connects the outlet fitting 20 toan outlet coolant header 28. Each stator bar 16 includes a plurality ofhollow and solid copper strands respectively disposed in side-by-sideand superposed relation one to the other. The fitting, for example,fitting 20, includes a clip 30 formed of an electrically conductivematerial, such as copper. The clip comprises a body having a rectilinearopening at one end for receiving the strands of the stator bar 16. Atthe opposite end, there is provided an opening which in use is normallyengaged with a copper tube 32 which serves as both an electricalconnection and a hydraulic connection for flowing liquid coolant, e.g.,deionized water, into or from the chamber 34 defined by the stator barclip 30 and the exposed ends of the hollow and solid copper strands. Theliquid in the chamber either flows into the fitting and through thehollow strands for cooling purposes when the fitting comprises an inletfitting or receives the liquid coolant from the hollow strands foregress when the fitting is employed as an outlet fitting. As mentionedabove, the solid and hollow strands are brazed to one another, and acover is brazed to clip window. These junctions are among the potentialsites for leaks over time.

In an embodiment of the invention, the interior of the stator bar clip30, including the corrosion susceptible braze joint between the clip 30and the stator bar 16, and the cover to clip braze seal, areelectroplated so that a thin metallic layer is deposited on the entireinterior surface. The metallic layer provides a corrosion resistantbarrier coating to prevent water access to corrosion and/or leaksusceptible region(s) such as the clip-to-strand braze joints and thecover-to-clip braze seal.

Electroplating is facilitated by isolating the target stator bar clip,including the braze joint between it and the stator bar, such as bypartial disassembly to provide access to the end of the clip remote fromthe stator bar. Referring to FIG. 2, a suitable assembly forelectroplating the stator bar clip is schematically illustrated by wayof example. In the illustrated embodiment, plastic tubing 40 is securelycoupled to the clip and extends up and away from the clip to define aplating solution bath upper surface 42 whereas the plating solution 44will fill the chamber 34 defined by the stator clip. A pump 46 isprovided to circulate plating solution 44 through the bath defined bythe interior of the stator clip 30 and the interior of the tubing 40.The plating solution may be initially loaded to the clip interiorthrough the tubing 40, or the pump can load the bath through inflow tube48. In the latter case, a reservoir of plating solution (not shown) isprovided and operatively coupled to the pump. In either case,recirculating flow tube 50 recirculates fluid to the pump (andreservoir, if any) and removes any gas, such as hydrogen bubbles,trapped against the uppermost portion of the clip interior. Theresultant circulating flow advantageously promotes agitation.

In the embodiment illustrated in FIG. 2, the anode 52 is a length of 10gauge insulated copper wire stripped for several inches and rolled intoa coil to reduce its overall dimensions. Of course, a copper wire isjust one example of a consumable metal source that may be provided asthe anode. Other alternatives include a metal mesh structure. Electricalcontact between the anode and the component being plated should ofcourse not occur. Therefore, in an exemplary embodiment the anode isinserted into, e.g., a double walled porous bag 54 of non-conductivematerial to prevent electrical contact but to allow solution access. Theanode is connected to a suitable poser supply (a 2-3 V DC power supplywas used in a test apparatus). The device itself is electrically coupledto the power supply as a cathode, e.g., via contact 56.

In the assembly illustrated in FIG. 2, the reservoir may be closedremote from the access tube. In the alternative, a recirculating systemmay be provided wherein the remote end is not closed, but instead theflow is through the clips 30 at each end of the stator bar(s), and theinterior of both clips and the respective braze joints are platedsimultaneously, as schematically illustrated in FIG. 3. In theillustrated embodiment, plating solution reservoirs 60, 62 are disposedin flow communication with each clip, and a pump 64 is provided tocirculate the plating solution through the clips and bar 16.

Although not illustrated in detail, in practice the device is heated,e.g. with heating tape, to between 40 and 50° C. Heating may also beaccomplished with an in-line heater in the electrolyte return tube.

An exemplary acid copper electroplating solution is a mixture of e.g.,water, sulfuric acid, copper sulfate, and a trace of hydrochloric acid.To this mixture a number of organic constituents are added that serve toregulate and distribute the delivery of, e.g., copper to the surfacebeing plated. The two basic organic additives are commonly referred toas the “brightener/leveler” and the “carrier”.

An electroplating cell is typically comprised of a (non-metallic)container full of the electroplating solution in flow communication withthe area or component surface to be treated and a source of platingmetal ions, as the anode. In the illustrated embodiment, theelectroplating cell is comprised of the component (clip 30) interior 34,the access tubing 40 interior volume, the inflow and outflow tubes48,50, and the remote reservoir for the solution (if provided) and, asthe anode, the coiled copper wire 52, or other consumable metal source,disposed within the access tubing 40 and/or clip 30. This ion supplymust be capable of continuous sourcing into a near short circuit load. Atypical copper electroplating bath has an effective full load operating“impedance” that ranges from 0.025 Ohms and 0.015 Ohms. The surface forreceiving the electroplated coating, referred to as the cathode, in thiscase the clip, the stator bar, and the braze joint therebetween, isconnected to the negative terminal of the current source. Deposition of,e.g., copper in this example, on the target component occurs when anelectrical potential is established between the anode and the cathode(target component). The resulting electrical field initiateselectrophoretic migration of copper ions from the anode to theelectrically conductive surface of the cathode, where the ionic chargeis neutralized as the metal ions plate out of solution. As a result, auniform, thin coating of smooth, bright copper is deposited on thetarget surface(s).

It should be noted that if it is anticipated that air or evolved gas maybecome trapped, e.g., in a crevice or a blind recessed area, it willprevent plating in such areas and pressure treatment may be necessary.Pressure treatment may be applied by defining the water pass system as aclosed loop that can be sealed and pressurized. Additionally, locatingthe return flow tube to draw fluid from the vertically upper portion ofthe stator clip, as mentioned above, facilitates the removal of trappedair or evolved gas to facilitate uniform plating.

The electroplating process is continued until a deposit layer of about 1to 3 mm is achieved. The process can be ceased at any time simply bydisengaging the power source.

As an alternative to electroplating a single metal, the plating of thebraze joint may be accomplished as a double-layered plating with a firstplating followed by a second plating applied over the first platinglayer. As will appreciated, a double layered plating provides enhanceddurability as compared to a single layered plating particularly wherethe plating coatings have selective properties; for example, anunderlying plated film having strong corrosion resistance and a secondplated film applied for air tightness, durability and corrosionresistance.

As a further alternative, the water pass of the stator bars and/or theclips, for example, may be plated before assembly and then plating canagain be carried out after assembly to plate the braze joints, toeffectively provide a double layered plating in certain areas or oncertain parts.

In accordance with this invention, all of the joints which have thepotential for forming a leakage path are electroplated. In this way, theexisting leak or leaks at the stator bar end connections of a generatorin the field are repaired. Additionally, by providing a protectivecoating to all potential leakage paths of the joints, a seal is providedwhich will ensure against the formation of leakage paths in the future.That is, the electroplating not only isolates the liquid coolant fromthe brazing material and seals between the joints of adjacent strandsand the outermost strands, but can be used to deposit a layer ofmaterial over, to seal, the entire interior surface of the coolant flowpath, whereby all potential leakage paths are sealed to prevent futureleaks. While the present invention is particularly applicable to fieldrepairs of existing generators, it may also be applied to generatorsduring initial manufacture to provide protection against future leakage.

As noted above, since the method disclosed herein is capable of applyinga coating over the entire wetted surface of the flow system, the methodcan address the clip crevice corrosion leak issue and also other leakissues in liquid cooled generators. For example, the method embodyingthe invention may used to address clip crack leaks, clip window leaks,plumbing and fitting leaks, and connection ring leaks. Thus, while theinvention has been described in connection with what is presentlyconsidered to be the most practical and preferred embodiment, it is tobe understood that the invention is not to be limited to particulars ofthe disclosed embodiment, but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

1-11. (canceled)
 12. A method of sealing interior surfaces of a fitting receiving a stator bar end, to define a seal substantially impermeable to liquid, the fitting having a chamber for receiving a liquid through an opening in the fitting and said chamber being in communication with hollow strands forming at least a portion of said stator bar, said method comprising the steps of: disposing an electroplating solution in said chamber; disposing an anode of the metal or metal alloy to be deposited in said electroplating solution; connecting said anode to the positive terminal of a current source as a cathode; electrically connecting said stator bar to the negative terminal of said current source; and establishing an electrical potential between said anode and said cathode to initiate migration of metal ions from the anode to the electrically conductive inner surface of the component.
 13. A method as in claim 12, wherein said anode is formed from at least one of nickel, chromium, zinc and copper.
 14. A method as in claim 12, wherein said anode comprises a consumable metal source.
 15. A method as in claim 12, further comprising pumping electroplating solution into said chamber and drawing electroplating solution out of said chamber, to thereby circulate electroplating solution through said chamber.
 16. A method as in claim 15, wherein said electroplating solution is pumped into said chamber from a reservoir of electroplating solution.
 17. A method as in claim 12, wherein said disposing an anode comprises disposing an anode in said chamber.
 18. A method as in claim 12, further comprising coupling an access tubing to an inlet end of said component for defining an electroplating bath with said interior of said component.
 19. A method as in claim 18, wherein said disposing an anode comprises disposing a wire formed from the metal or metal alloy to be deposited through the access tubing into the electroplating bath.
 20. A method of sealing a stator bar with an interior liquid pass for flow of coolant forming at least a portion of said stator bar, comprising electroplating at least a portion of an interior wetted surface of said stator bar. 